Solvation and halogeno complexation of the cadmium(II) ion in hexamethylphosphoric triamide

Abstract

The complexation of the cadmium(II) ion with bromide and iodide ions has been studied by calorimetry in hexamethylphosphoric triamide (HMPA) containing 0.1 mol dm^–3(n-C₄H₉)₄NClO₄ as a constant ionic medium at 25 °C. The formation of [CdX_n]^(2–n)+(n= 1, 2, 3, 4 for X = Br; n= 1, 2, 3 for X = l) is revealed, and their formation constants, reaction enthalpies and entropies were determined. The ternary Cd^II–Br–I system has also been examined and the formation of the [CdBrI]⁰ and [CdBr₂I]^– complexes is elucidated. The Cd^II–X systems exhibit unusual thermodynamic behaviour that is not observed in the M^II–X (M = Co, Zn) systems. The reaction enthalpies very in the order, ΔH^°₁ > ΔH^°₂≈ΔH^°₄ > ΔH^°₃ in the Cd^II–Br system, while ΔH^°₁ < ΔH^°₂ < ΔH^°₃ < ΔH^°₄ in the Zn^II–Cl system. The ΔS^°₁ values, 179 and 187 J K^–1 mol^–1 for the formation of [CdBr]⁺ and [CdI]⁺, respectively, are significantly larger than the corresponding values in the Co^II and Zn^II systems. These facts strongly suggest that the cadmium(II) ion has a larger solvation number in HMPA. EXAFS spectroscopy provided evidence that the cadmium(II) ion is coordinated to five HMPA molecules, whereas cobalt(II) and zinc(II) ions are coordinated to four HMPA molecules.